Method of determining cationic surfactant

ABSTRACT

A method of determining a cationic surfactant in a sample solution, characterized by passing the sample solution through a disk containing styrene/divinylbenzene copolymer particles and a disk containing an anion-exchange resin; and then subjecting the resultant solution to high-performance liquid chromatography. The method of the present invention enables separation/quantification of CS in river water or lake water which also contains inorganic ions and AS of high concentration, even when the CS content is below the order of ppb.

TECHNICAL FIELD

The present invention relates to a method for correctly and rapidlydetermining cationic surfactants in an aqueous solution such asenvironmental water, even when the cationic surfactant coexists with ananionic surfactant in the solution; and to an apparatus employed in themethod.

BACKGROUND ART

Cationic surfactants (CS) have widely been employed as primarycomponents of, for example, a hair rinse, a hair treatment agent, or asoftening agent for clothing. However, since CS is not rapidlydecomposed by microorganisms because of its sterilizability, concern hasarisen about the effect of CS on aquatic living organisms. Accordingly,determination of CS in environmental water is very important, anddevelopment of a method for rapid analysis of low-concentration CS isdemanded.

In order to establish a highly sensitive separation/quantificationmethod for CS in environmental water by means of high-performance liquidchromatography (HPLC) employing an electrical conductivity detector, thepresent inventors have conducted studies on HPLC systems employing ahydrophilic polymer gel column serving as a separation column. As aresult, the present inventors have developed a highly sensitive HPLCsystem which provides a detection limit of 0.03 to 0.04 μM, which is onthe order of {fraction (1/10)} or less compared with that of aconventional HPLC system employing an electrical conductivity detectoror an indirect UV absorption detector.

However, the concentration of CS present in river water is considered tobe on the order of ppb or less, and therefore, determining the CScontent requires concentration of CS in a sample and removal ofinorganic ions and anionic surfactants (AS).

In view of the foregoing, an object of the present invention is toprovide a method for accurately determining the CS concentration in anaqueous solution containing both AS and CS such as environmental water.

DISCLOSURE OF THE INVENTION

In order to attain the above object, the present inventors haveperformed extensive studies on disks containing a variety of resins thatcan separate AS from CS. As a result, they have quite unexpectedly foundthat, when a disk containing styrene-divinylbenzene copolymer particlesis employed in combination with a disk containing an anion-exchangeresin, AS is selectively removed from the solution and CS isconcentrated, and that when the resultant solution is subjected tosolid-phase extraction, if necessary, and then subjected to HPLC, theamount of CS can be accurately determined. The present invention hasbeen accomplished on the basis of these findings.

Accordingly, the present invention provides a method for quantifyingcationic surfactants in a sample solution, which comprises causing thesample solution to pass through a disk containing styrene-divinylbenzenecopolymer particles and a disk containing an anion-exchange resin; andsubsequently subjecting the resultant solution to high-performanceliquid chromatography.

The present invention also provides a method for quantifying a cationicsurfactant in a sample solution, which method comprises causing thesample solution to pass through a disk containing styrene-divinylbenzenecopolymer particles and a disk containing an anion-exchange resin, andthrough a solid-phase extraction column; and subsequently subjecting theresultant solution to high-performance liquid chromatography.

The present invention also provides an apparatus for measuring acationic surfactant in a sample solution, the apparatus comprising adisk containing styrene-divinylbenzene copolymer particles, a diskcontaining an anion-exchange resin, and a high-performance liquidchromatograph.

The present invention also provides an apparatus for measuring acationic surfactant in a sample solution, the apparatus comprising adisk containing styrene-divinylbenzene copolymer particles, a diskcontaining an anion-exchange resin, a solid-phase extraction column, anda high-performance liquid chromatograph.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation showing an on-line solid-phaseextraction/HPLC system employed in the present invention.

FIG. 2 shows the results of separation of CS in a river water sample,the CS separation having been performed through preliminary treatment byuse of SDB-XD (disk containing SDB particles) and ANION-SR (diskcontaining anion-exchange resin), on-line solid-phase extraction, andHPLC.

FIG. 3 shows the results of standard addition test employing CTMA(cetyltrimethylammonium ion) and TMSA (trimethylstearylammonium ion) inseparation/quantification of CS in a river water sample, the CSseparation/quantification having been performed through preliminarytreatment by use of SDB-XD and ANION-SR, on-line solid-phase extraction,and HPLC.

Reference numerals shown in FIG. 1 denote the following items.

-   1: Sample solution-   2: Pure water-   3: Pump-   4: Solid-phase extraction column-   5: Syringe-   6: Pump-   7: Eluent-   8: Waste-   9: Separation column-   10: Suppressor-   11: Detector-   12: Regeneration solution

BEST MODE FOR CARRYING OUT THE INVENTION

No particular limitations are imposed on the sample solution employed inthe quantifying method of the present invention, so long as the samplesolution is an aqueous solution containing CS. Specific examples of thesample solution include environmental water such as river water or lakewater. There has not yet been developed a suitable method fordetermining CS in river water or lake water, which contains, in additionto CS, large amounts of AS and inorganic ions.

In the method of the present invention, firstly, a sample solution iscaused to pass through a disk containing styrene-divinylbenzenecopolymer (SDB) particles and a disk containing an anion-exchange resin.When the sample solution is caused to pass through these disks, AScontained in the solution is removed, and CS is selectively collected.The finding that merely a combination of the above two disks, amongnumerous disks, enables removal of AS and high-efficiency collection ofCS is a quite unexpected one. Each of these disks is prepared by fixingfiller particles onto fibrous matter, which preferably comprises afluorine-containing resin, such as Teflon (registered trademark). Theanion-exchange resin is preferably a quaternary-ammonium-salt-containinganion-exchange resin. These disks may be Empore Disk SDB-XD and EmporeDisk ANION-SR (trade names), which are available from 3M. No particularlimitations are imposed on the sequence in which these disks are used.

Subsequently, the sample which has passed through these disks is, ifnecessary, caused to pass through a solid-phase extraction column, andthen subjected to high-performance liquid chromatography (HPLC). Whenthe sample is subjected to solid-phase extraction, CS in the sample canbe highly concentrated, and sensitivity for determining the CS contentby means of HPLC is drastically enhanced. The CS concentration ispreferably carried out by means of on-line solid-phase extraction.

As shown in FIG. 1, on-line solid-phase extraction is carried out bysupplying pure water 2 and a sample solution 1 to a solid-phaseextraction column 4 by use of a pump 3, and by subsequently circulatingthe sample solution through the column a predetermined number of times.The solid-phase extraction employs a solid phase filled with ahydrophilic polymer; for example, a solid phase filled with a highlycross-linked polyvinyl alcohol gel. Examples of thepolyvinyl-alcohol-gel-filled solid phase include Shodex AsahipakGF-310HQ (Showa Denko K. K.).

The sample which has undergone the CS concentration or the sample whichhas passed through the aforementioned disks is subjected to HPLC. Asshown in FIG. 1, the system for HPLC includes a separation column 9, asuppressor 10, and a detector 11. The separation column is preferably acolumn filled with a hydrophilic polymer; for example, a column filledwith a highly cross-linked polyvinyl alcohol gel. Specific examples ofthe column include Shodex Asahipak GF-310HQ (Showa Denko K. K.).Examples of the suppressor to be employed include a suppressor employingan anion-exchange membrane, such as DIONEX CMMS-II (Nippon Dionex K.K.).

Examples of the detector 11 include an electrical conductivity detector,an indirect UV absorption detector, and a mass spectrometric detector.The detector 11 is particularly preferably an electrical conductivitydetector or a mass spectrometric detector. In the case where a massspectrometric detector is employed, even when solid-phase extraction isnot carried out before subjecting HPLC, CS of very low concentration canbe quantified. Meanwhile, in the case where an electrical conductivitydetector is employed, preferably, solid-phase extraction is carried outbefore subjecting HPLC.

The method of the present invention can determine the CS level of riverwater or lake water which also contains AS in an amount more than 100times that of the CS, even when the CS content is below the order ofppb.

EXAMPLE

The present invention will next be described in more detail by way ofExample, which should not be construed as limiting the inventionthereto.

Example 1

(1) An on-line solid-phase extraction/HPLC system as shown in FIG. 1 wasemployed. A solution mixture of 4,4′-dipyridyl, hydrochloric acid, andacetonitrile was employed as an eluent 7, and Shodex Asahipak GF-310HQ(4.6 mm I.D.×150 mm, 6 μm) was employed as a separation column 9. As asolid-phase extraction column 4, there was employed a column (4.6 mmI.D. ×10 mm, 9 μm) filled with a filler which is formed of the same basematerial as that of the filler of the separation column, but has aparticle size different from that of the separation column filler.DIONEX CMMS-II (Nippon Dionex K. K.) was employed as a suppressor 10,and a tetrabutylammonium hydroxide solution was employed as aregeneration solution 12. As a sample 1, the following three cationspecies were employed: cetyltrimethylammonium (CTMA) ion,tetradecyldimethylbenzylammonium (TDDBA) ion, andtrimethylstearylammonium (TMSA) ion. The following types of Empore Diskwere employed: SDB-XD (disk containing SDB particles), CATION-SR (diskcontaining cation-exchange resin), ANION-SR (disk containinganion-exchange resin), and SDB-RPS (disk containing sulfonated SDBparticles).

(2) Firstly, the CS concentration was regulated to 50 nM, andcollection/concentration of CS by Empore Disk was evaluated.Specifically, the sample solution was caused to pass through each of theabove disks, and then the CS content of the resultant solution wasdetermined by use of the on-line solid-phase extraction/HPLC systemshown in FIG. 1. In order to prevent adsorption of the CS onto the wallof a glass container, hydrochloric acid was added to the sample solutionsuch that the hydrochloric acid concentration became 0.1 M. The resultsof the CS content determination revealed that, in terms of CS collectionefficiency, SDB-RPS is superior to SDB-XD, and SDB-XD is superior toANION-SR. However, when AS (5 μM) was added to the sample solution, eachof the disks exhibited considerably lowered percent collection of theCS. In an effort to attain high percent collection of the CS, extensivestudies were performed. The results of the studies revealed that whenSDB-XD (disk containing SDB particles) is stacked on ANION-SR (diskcontaining anion-exchange resin), and the sample solution is caused topass through the thus-stacked disks, AS is removed, and the percentcollection of the CS becomes high (95% or more). Such high percentcollection was not attained by means of other combinations of the disks.Even when SDB-XD and ANION-SR were stacked in an inverse manner, similareffects were obtained.

(3) On the basis of the results of (2) above, the method of the presentinvention was applied in practice to river water. FIG. 2 shows achromatogram of a river water sample collected from the Ebi Riverflowing through Funabashi City. CS in the sample was identified throughoverlap injection, since the retention time of the CS is affected bysubstances coexisting with the CS. As s result, CTMA and TMSA were foundto be present in the sample. Mass spectrometry of the sample alsoconfirmed the presence of CTMA and TMSA. When the sample was subjectedto standard addition test for determining the amounts of these CSs, goodlinear relations were obtained as shown in FIG. 3. On the basis of theresults, the amounts of CTMA and TMSA contained in the river watersample were calculated as 1.7 nM (0.54 ppb) and 7.0 nM (2.5 ppb),respectively.

INDUSTRIAL APPLICABILITY

The method of the present invention enables separation/quantification ofCS in river water or lake water which also contains inorganic ions andAS of high concentration, even when the CS content is below the order ofppb.

1. A method for quantifying a cationic surfactant in a sample solution,which method comprises causing the sample solution to pass through adisk containing styrene-divinylbenzene copolymer particles and a diskcontaining an anion-exchange resin; and subsequently subjecting theresultant solution to high-performance liquid chromatography.
 2. Aquantifying method according to claim 1, wherein the sample solution isan aqueous solution containing a cationic surfactant and an anionicsurfactant.
 3. A quantifying method according to claim 1 or 2, whereindetection means employed in the high-performance liquid chromatographyis mass spectrometry.
 4. A method for quantifying a cationic surfactantin a sample solution, which method comprises causing the sample solutionto pass through a disk containing styrene-divinylbenzene copolymerparticles and a disk containing an anion-exchange resin, and through asolid-phase extraction column; and subsequently subjecting the resultantsolution to high-performance liquid chromatography.
 5. A quantifyingmethod according to claim 4, wherein the sample solution is an aqueoussolution containing a cationic surfactant and an anionic surfactant. 6.A quantifying method according to claim 4 or 5, wherein detection meansemployed in the high-performance liquid chromatography is electricalconductivity analysis or mass spectrometry.
 7. An apparatus formeasuring a cationic surfactant in a sample solution, the apparatuscomprising a disk containing styrene-divinylbenzene copolymer particles,a disk containing an anion-exchange resin, and a high-performance liquidchromatograph.
 8. An apparatus for measuring a cationic surfactant in asample solution, the apparatus comprising a disk containingstyrene-divinylbenzene copolymer particles, a disk containing ananion-exchange resin, a solid-phase extraction column, and ahigh-performance liquid chromatograph.